Kimchi not very fermented, did not expand or get juicy

In general, slimy brine is indicative of a fermentation problem. Your brine might be too weak (what concentration did you use?), it may be too warm, your brine might not cover the vegetables, there might be some air bubbles trapped in your ferment.

This article suggests that in the early days of sauerkraut fermentation, the concentration of slime forming bacteria - Leuconostoc - is relatively high and towards the end of fermentation it's too small to measure. It doesn't say anything about slimy brine, but it's possible that the brine is temporarily slimy in the early days of fermentation and as the acidity increases it dissipates. I've never checked a ferment this early, so I can't comment on it.

Personally, I would let it continue to ferment for a week or two and then see whether the perceived sliminess has dissipated or increased. It's possible that it's too early to tell. Slimy vegetables, without a doubt, should be discarded.

References

• http://www.culturesforhealth.com/fermented-vegetables-troubleshooting-guide

If your jars aren't in the refrigerator already, I highly recommend unscrewing the lids as soon as possible...unless you want to be able to share stories about how you found glass shards and the smell of kimchi everywhere in your kitchen one day.

Depending on when you mean to eat them, I'd recommend a mix of room temperature ripening and fridge storage. Keep the lid completely unscrewed while at room temp until ripened to a stage you prefer, and keep it loosely on in the fridge and eat it up in the next few weeks (where it will continue to ferment some more but slow down drastically). It's not a terrible idea to put something like a sheet pan under the jars just in case one overflows.

Cheesecloth is overkill, but you could choose to use that or anything that'll cover the top of a jar without making it airtight while at room temp. Glass jars are used often in commercial kimchi production and sale, so other than making sure the lid isn't fully screwed on they're fine to use. The explosion concern is mostly when you leave it too long (more than several hours?) at room temp or in the fridge for more than a couple weeks - while the jar is completely sealed.

This all depends on the kind of kimchi you are storing, of course. I'm assuming it's the freshly made napa cabbage type you're talking about. Some kimchis don't ferment as much or at all.